One very important source of helium is as a component in the gas stream from a natural gas reservoir. Because of the wide difference in the volatility of natural gas, which is essentially methane, and the volatility of helium, it is relatively easy to separate helium from natural gas.
Often, however, the gas stream from a natural gas reservoir also contains a significant amount of nitrogen. The nitrogen may be naturally occurring and/or may have been injected into the reservoir as part of an enhanced gas recovery or enhanced oil recovery operation. In this situation the gas stream from the reservoir, after certain precleaning operations to remove acid gases, water, and/or higher hydrocarbons, is passed to a nitrogen rejection unit or NRU wherein the methane is separated from the nitrogen by cryogenic rectification. The nitrogen fraction may comprise from 10 to 70 percent of the feed to the NRU. Due to the relative volatilities of these gases, the helium in the NRU feed is concentrated with the nitrogen. In order for the helium to be of commercially acceptable purity the helium-nitrogen mixture from the NRU must be separated.
This separation is relatively difficult and costly and consequently a significant portion of the helium is lost by remaining with the nitrogen.
It is therefore an object of this invention to produce helium gas from a nitrogen rejection unit simply and efficiently with high recovery.